Elevator systems have long utilized safety chains of hard-wired relay contacts which must essentially all be made prior to moving the elevator car. Early relays used for the safety chain were DC because AC relays were not as reliable. Since installed elevator systems typically have a very long life span, even longer than the lifespan of then available commercial DC relays, many elevator companies chose to make their own DC relays to better control reliability and quality.
However, as commercial AC relays have improved in reliability, and the cost of maintaining an in-house DC relay manufacturing capability has increased, elevator companies have gradually shifted to use of AC relays for circuits in which hard-wired logic is still desirable or necessary, such as in safety chains. Most US domestic companies have chosen to use 220 Volt, 60 cycle AC for such relay logic circuits, although at least one well-known company has selected 110 VAC which, although presenting less of a voltage hazard, creates a voltage drop problem in long wire runs with numerous hookup points and inductive losses. [Current systems employing combined door and safety chains present a problem regarding the cumulative voltage drop over large distances of wire (that are run throughout the hoistway and travelling cable, etc.) with numerous hookup points. The voltage must be of sufficient magnitude to supply a voltage capable of sustaining a relay coil.]
An early microprocessor-based elevator system of the Otis Elevator Company utilized DC relay logic in the safety chain and had a "DST" (Door Status) relay which was energized when all the door contacts in the door portion of the safety chain were made. A blocking diode was used to obtain isolation of the relay from the door chain bypass circuit and the rest of the safety chain. (Such a bypass circuit is designed to short-circuit the door portion of the safety chain when the car is in a door zone and moving at a slow rate). In this way, a contact of the DST relay may be sampled by the microprocessor-based controller. A check was made of the DST contact upon receiving a door fully open indication. The necessary circuit isolation for the DST relay (a blocking diode) was made possible by the unidirectional quality of the DC supply.